A flat spiral coil, or pancake coil, is a common electrical device often used for sensing, modulating or creating electric and magnetic fields. Generally, when assembling a flat spiral coil, wire is drawn through an epoxy resin bath, so that the resin coats the outside of the wire, before the wire is wound into the flat spiral shape on a substrate. As the epoxy resin cures it creates a bond with the substrate which holds the flat spiral coil in position and keeps its shape. This technique works well for coils created and used at or near room temperature.
For many applications, however, colder temperatures are required. For example, superconductivity requires cryogenic temperatures. In many cases, winding a flat spiral coil from superconducting wire can be useful, allowing, for example, much more sensitive instruments to be built than is possible with non-superconducting wire. In such highly sensitive applications, geometric stability is a concern and large changes in temperature caused by cooling a coil to superconducting temperatures results in thermal contraction of the wires, substrate and epoxy resin creating stresses, and straining or warping of materials. In addition, when using an epoxy resin to bond a superconducting coil to a substrate and subsequently cooling it to cryogenic temperatures, differential thermal contraction frequently causes shear forces greater than the epoxy-substrate bond can sustain, resulting in delamination of the coil.
One approach to solving this problem is to attempt to match the coefficients of thermal expansion of the wire, substrate and epoxy. However, while it is sometimes possible to match two of these closely, matching all three is often very difficult. Even if it can be achieved, it often requires undesirable trade-offs in other material properties, such as thermal conductivity or workability of materials.